Human race needs sustainable energy to develop. The ever-increasing carbon emissions from the use of fossil fuels and human conflicts call for the exploitation of alternative and renewable energy sources such as solar. Among emerging photovoltaics, dye-sensitized solar cells (DSSCs), amidst other thin-film technologies, have captured interests from academia and industry thanks to their low production costs, decent stability in applications, high efficiency and their tuneable optoelectronic properties. A DSSC layout comprises a photosensitizer immobilized on a semi-conductor, a redox couple (usually I3-/I-) and a Pt-coated cathode. Among these components, the photosensitizer allows for the fabrication of chromatic and semi-transparent solar cells, which are appealing candidates in the Building Integrated PhotoVoltaics (BIPV) industry. Nevertheless, for application as windows in BIPV, transparency and efficiency have to be optimized. Among the different approaches reported in this field, the use of NIR-photosensitizers allowed to reach an impressive transparency in the visible up to 76% but lead to sacrificing efficiencies. Another approach consists in developing semi-transparent solar cells with visible light-absorbing dyes but the optical transmission in that case is fixed during manufacturing and a compromise has to be found between efficiency and transparency. The STEP team has made efforts to reconcile these properties by replacing classical photosensitizers with organic photochromes. We have developed a new class of DSSCs with dynamic optical properties, capable to self-adapt their transparency levels and energy production as a function of daylight termed photo-chromo-voltaic devices. To attain high performance solar cells to pave their way for application, optimization of working conditions and hence the dyes is necessary. In this PhD, I will report on the development of photochromic solar cells registering efficiencies exceeding 4.2%, unravel the structure-property relationships and the photochromism-photovoltaic interplay for this new class of molecules. With this understanding, I have presented DSSCs with efficiencies ~5% marking the state-of-the-art in this subject area and suggested future research directions.

Contact : Renaud Demadrille